I guess your server is back online.
I still couldn’t use humann_databases --download, but wget now works.
For those out there also struggling to download the database, I did this:
mkdir chocophlan
cd chocophlan
wget --no-check-certificate http://huttenhower.sph.harvard.edu/humann_data/chocophlan/full_chocophlan.v201901_v31.tar.gz
tar -xf full_chocophlan.v201901_v31.tar.gz
humann_config --update database_folders nucleotide /FULL/PATH/chocophlan
And, same for uniref90:
mkdir uniref
cd uniref
wget --no-check-certificate https://huttenhower.sph.harvard.edu/humann_data/uniprot/uniref_annotated/uniref90_annotated_v201901b_full.tar.gz
tar -xf uniref90_annotated_v201901b_full.tar.gz
humann_config --update database_folders protein /FULL/PATH/uniref